In generative manufacturing methods, in which a three-dimensional object is manufactured layer-wise by sintering or melting a building material in powder form, plastic powders play an important role as starting materials as such materials are advantageous for the process due to their low melting points and small heat conductivities. Usually, the energy that is necessary for the sintering or melting process is introduced by means of a laser. However, to some extent also electron beams or infrared beams are used for the same purpose.
DE 197 47 309 A1 describes a polyamide 12 powder of the prior art, which powder is particularly useful for selective laser sintering.
DE 10 2006 053 121 B3 describes a laser sintering device as device for manufacturing a three-dimensional object, which is shown in FIG. 4. The build-up process takes place in a container 1, which is open to the top. A support 2 for supporting the three-dimensional object 3 to be built is provided in the container. By a drive 4 the support 2 can be moved up and down in a vertical direction A in the container 1. The upper edge of the container 1 defines a build-up zone 5. An irradiation device 6 in the form of a laser is arranged above the build-up zone 5, which irradiation device emits a directed laser beam 18, that is directed at the build-up zone 5 by means of a deflection device 7. Further, an application device 40 (sometimes also named “re-coater”) is provided for applying a layer of a material in powder form, which material is to be solidified, onto the surface of the support 2 or onto the most recently solidified layer. The application device 40 can be moved bidirectionally across the build-up zone 5 by means of a drive, which is schematically indicated by the arrows B. The application device is fed from two powder reservoirs 10 by means of two dosage devices 9 at the left and at the right of the build-up zone. Furthermore, two overflow containers 11 are provided at the left and at the right of the build-up zone, which overflow containers are able to accommodate the surplus powder that accumulates during the application of powder.
A heating device 12 for heating the powder bed 19 and in particular for pre-heating an applied powder layer that has not yet been sintered (solidified) to a suitable build temperature, is arranged above the build-up zone 5. For example, the heating device 12 is constructed in the form of one or more radiant heaters such as infrared heaters. The heating device is arranged above the build-up zone 5 such that an applied powder layer can be uniformly heated.
A temperature measuring device 13 such as a pyrometer or an IR camera is provided above the build-up zone 5 at a distance to it. By the temperature measuring device the temperature of the most recently applied powder layer can be measured.
A process chamber 16 secludes the build-up zone from the surrounding. Thus, it is possible to carry out a build process in the absence of air and to prevent an oxidation of the powder.
An open-loop control and/or a closed-loop control 17 serves for an open-loop control and/or a closed-loop control of the movement B of the application device 40. This open-loop control device and/or closed-loop control device also makes an open-loop control/closed-loop control of the movement A of the support 2, of the power of the heating device 12, of the power of the irradiation device 6 and of the deflection by the deflection device 7. To this effect the open-loop control device/closed-loop control device 17 is connected to the drive of the application device 40, the drive 4, the heating device 12, the temperature measuring device 13, the deflection device 7 as well as to the irradiation device 6.
In the following the operation of such a laser sintering device will be described:
At first, the application device 40 is positioned below the dosage device 9 and is filled with an amount of material in powder form for one layer from the reservoir 10.
Afterwards a powder layer is applied onto the support 2 or a most recently solidified layer by moving the application device 40 parallel with the surface of the build-up zone 5. In the process, material in powder form is replenished from the application device. After the application of a layer of the material in powder form, a solidification is carried out at those positions of the layer that correspond to a cross-section of the object, wherein the layer is exposed to the light of the laser beam 18. After the solidification of one layer, the support 2 is lowered by a distance that corresponds to the thickness of one layer and the above-described steps are repeated until the manufacturing of the three-dimensional object 3 is completed.
During the build process the object to be generated rests inside of the powder bed of unsolidified powder that is surrounding it, whereby the object is supported by such powder. As a result, at the completion of a build process usually there are substantial amounts of unsolidified powder. Therefore, it is desirable to re-use as much as possible of such so-called “old powder” for a further build process. However, as the unused powder was exposed to high temperatures immediately below its melting point for long periods of time during a build process, there is the problem that due to these environmental conditions the powder may have been subject to an aging process, in which aging process it could be damaged thermally and/or thermooxidatively. This leads to the situation that for further build processes the old powder has to be mixed with fresh powder, wherein the proportion of fresh powder usually has to be at least 50%. The percentage of fresh powder often is also designated as “reconditioning rate”, “renewal rate” or “refresh rate”.
DE 103 30 590 A1 concludes that this aging process may be explained by a post-condensation:
Under the environmental conditions during a build process, the free carboxy and amino end groups in polyamide 12 react with each other under dehydration (the actual post-condensation) also in the unused powder. Furthermore, in DE 103 30 590 A1 it was observed that the loss of amino groups is more than stoichiometric in relation to carboxy groups. It was assumed that a cause for this would be a thermooxidative elimination of the amino groups with subsequent cross-linking. In each case the aged powders showed a remarkable increase of the solution viscosity.
In order to increase the re-usable portion of old powder, DE 103 30 590 A1 suggests to use for the build process always a polyamide, in which there is a ratio of carboxy end groups to amino end groups of at least 2:1 and in which the content of amino end groups is below 40 mmol/kg (regulated polyamide), in contrast to a so-called unregulated polyamide as it is for example described in DE 197 08 946. The desired end group ratio here is obtained by a carboxylic acid as regulator. Here, in DE 103 30 590 A1 only a small increase of the solution viscosity after an aging of the powder was also observed for powder mixtures of regulated and unregulated PA 12. Without giving any further reasons, a preferable range from 10-90% and an especially preferable range from 25-75% were cited for the proportion of a regulated polyamide.
It is important that the objects to be manufactured have advantageous mechanical properties, in particular a high elongation at break. In this regard, DE 103 30 590 A1 refers to an improvement of the mechanical properties of the manufactured part due to the use of regulated polyamide 12. However, irrespective of such fact, when carrying out a laser sintering of plastic powders, further boundary conditions have to be taken into consideration besides the mechanical properties of the object to be manufactured: On the one hand, attention has to be paid to a part warping that is as small as possible. On the other hand attention has to be paid to a quality of the outer surfaces of the object that is as high as possible.
Part warpage is a known problem. It is caused by a shrinking of semi-crystalline plastic materials during cooling down. A large change of volume during the solidification leads to an upward bending of the ends of the object. A non-satisfying surface quality of the outside part walls manifests itself in so-called depressions (sometimes also called dips), which are referred to as “sink marks” or “orange peel” and which can be particularly observed when old powder has been re-used.